One method for the preparation of dibutyltin dimethoxide involves the reaction of dibutyltin dichloride with sodium methoxide (Equation 1 ). This method of synthesis is described in U.S. Pat. No. 2,700,675 (Mack et al.) and is shown below: ##STR1## The crude dibutyltin dimethoxide is then purified by vacuum distillation. The relatively high cost of dibutyltin dimethoxide using the above process has prompted an investigation of alternative synthetic routes using relatively inexpensive starting materials.
Dibutyltin dialkoxides of higher alcohols have been synthesized by transesterification of dibutyltin dimethoxide with alcohols such as 1-dodecanol. This process is described in U.S. Pat. No. 2,727,917 (Mack et al.) and is shown below in Equation 2: ##STR2##
Also, dialkoxides of primary, alcohols have been synthesized via a two step reaction. The first step involves condensation of dibutyltin oxide with a primary alcohol in refluxing benzene or toluene at 80.degree.-110.degree. C., to produce 1,3-bis(alkoxy)tetrabutyl distannoxane. Water which is formed as a product in the condensation reaction is removed by azeotropic distillation. In the second step the distannoxane undergoes disproportionation at 180.degree.-220.degree. C. under reduced pressure to produce dibutyltin oxide and dibutyltin dialkoxide as shown below in Equation 3: ##STR3## It should be noted, however, that secondary alcohols do not react with dibutyltin oxide in this way.
The method illustrated by the reactions shown in Equation 3 does not work for methanol which does not form an azeotrope with water. Treatment of dibutyltin oxide with methanol and toluene at temperatures ranging from 115.degree.-180.degree. C. and pressures varying from ambient to 400 psi (2.756 MPa) results in the formation of 1,3 bis(methoxy)tetrabutyl distannoxane, but does not produce the desired dibutyltin dimethoxide in any significant yield.
An alternative route involving the reaction of dibutyltin oxide with dimethyl carbonate (in the presence of traces of methanol) to produce 1,3 bis(methoxy)tetrabutyl distannoxane exclusively is described in an article by Davies et al., entitled "Organic Chemistry, Pan XI--The Preparation of Organotin Alkoxides", Journal of Chemical Society, (C) (I971), pp. 3972-6. The reaction according to the Davies article is conducted in the presence of toluene at 80.degree.-110.degree. C. and 1 atmosphere pressure.
The present inventors have discovered that the highly desirable dibutyltin dimethoxide can be synthesized by reacting the 1,3 bis(methoxy)tetrabutyl distannoxane formed according to the Davies et al. article with methanol and dimethyl carbonate at elevated temperatures and pressures. Surprisingly, the present inventors also discovered that the addition of dimethyl carbonate as a reactant to a mixture of dibutyltin oxide and methanol, at elevated temperatures and pressures results in quantitative yields of dibutyltin dimethoxide along with the evolution of a quantitative amount of carbon dioxide.
Hence, the present inventors have discovered a unique low cost synthetic route to dibutyltin dimethoxide. Dibutyltin dimethoxide is a highly effective catalyst in numerous applications such as transesterification, esterification, ester interchange, transamination, and the synthesis of organic carbonates by the reaction of alcohols with alkyl carbamates and/or urea. The catalyst prepared by this technique may be used without purification thereby achieving an additional cost savings. This compound, as mentioned previously, is a starting material for the synthesis of dibutyltin alkoxides based on higher molecular weight alcohols as shown in Equation 2 above.
The present invention can also be used as a means of recycling and reactivating partially spent tin catalysts from the aforementioned applications by reconverting the inactive tin compounds to the dialkyltin dialkoxide by treatment with water followed by treatment with alcohol and dialkyl carbonate as described herein.
The present invention also provides many additional advantages which shall become apparent as described below.